1. Field of Invention
This invention relates to a method and apparatus for testing the integrity of an electrode in an electrode system for measuring an ion concentration or a redox potential in a solution, wherein in the circuit containing the electrode, a two directional electrical quantity is injected to determine the impedance of the electrode.
2. Discussion of the Prior Art
A conventional method and apparatus to carry out such testing is disclosed for example in U.S. Pat. No. 4,189,367 wherein the electrode system contains a high resistance ion selective membrane and a reference electrode. The impedance of the ion selective membrane is considerably greater than that of the reference electrode. For example, this may be by a factor 10 times greater than the combined impedance of the reference electrode and the solution. In practice, the impedance of the reference electrode is often about 10.sup.-3 to 10.sup.-5 times smaller than that of the high resistance ion selective electrode, which may be, for example, of glass.
In another example, U.S. Pat. No. 3,661,748 discloses a method and apparatus for testing the integrity of electrodes, wherein apart from electrodes for measuring concentrations of a gas, such as O.sub.2 or C0.sub.2 (which electrodes need a drving voltage and are not considered in this invention), a testing circuit is used for a series connection of a potentiometric electrode, such as a pH electrode, and a reference electrode. In this circuit, an AC voltage is injected in the series connection and phase-sensitive measuring is done of the AC current passing through the circuit. The AC voltage is applied to the system via an auxiliary electrode in contact with the liquid monitored by the ion sensitive electrode.
In both prior art disclosure, disadvantageously, the measuring circuit is maintained intact during the testing period. The result is that no individual testing of the ion sensitive electrode and the reference electrode is possible and that test values of each may be relatively inaccurate, whereas several types of failures may escape discovery, as may be apparent from the below discussion.
Apart from the high resistance membrane electrodes, such as glass electrodes, low impedance potentiometric electrodes exist, such as redox electrodes, which are selectively sensitive to for example Na, K or other ions.
When testing electrode systems, many types of electrode failures, faults and defects may occur. For example, apart from a short circuit originating from a crack in a glass electrode, for example, misreadings may be due to many other causes. Examples of other causes are:
(A) The connection with an electrode may be interrupted. In that case, its reading is highly constant, but may be in the region of normal readings.
(B) The electrode may be soiled or coated by deposits, so that its impedance increases and its sensitivity decreases, thus causing misreadings.
(C) The reference electrode may fail, due to loss of electrolyte, in which case its impedance will increase and its output may float, so that the value indicated by the electrode system will be false, but remain within the region of possible readings.
(D) The reference electrode is poisoned. For example, in case of a reference electrode of the type, metal-metal halide electrolyte, this may lead to a very strong increase of impedance, and even isolation of the electrode from the electrolyte, which will cause misreadings.
(E) The membrane of the reference electrode between its electrolyte and the liquid being monitored may become clogged, thus, leading to a high impedance of the reference electrode, which can even be put out of action.
In practice, failure to the reference electrode is a greater danger than the failure of, for example, the glass electrode. Furthermore, it is desired to be able to indicate not only the break down of a high impedance,but also other types of failures.
Thus, it can be appreciated that the prior art is replete with disadvantages and deficiencies.